Method of brazing



3,085,325 METHOD OF BRAZING Justin F. Slepitis, New City, N.Y., assignor to Radio Corporataon of America, a corporation of Delaware N Drawing. Filed Feb. 10, 1961, Ser. No. 88,265 4 Claims. (Cl. 29-493) This invention relates to improved alloys suitable for high temperature applications. More particularly, this invention relates to improved iron base alloys, and improved brazing methods utilizing the aforesaid alloys.

In many applications, such as electrical resistance heating elements, permanent molds for metal casting, die casting, carbonizing boxes, and the like, it is desirable to utilize alloys which will withstand high temperatures. Preferably the alloys utilized for such purposes should not only exhibit a high melting point and high tensile strength at elevated temperatures, but should also maintain their strength during repeated temperature cycling, exhibit dimensional stability, and be sufiiciently workable for economical fabrication. Furthermore, for commercial acceptance the alloys utilized should either have a low cost,

.or a useful life commensurate with their cost.

The three principal classes of alloys utilized for these high temperature applications have been nickel base alloys, for example an alloy consisting of about 80% nickel and 20% chromium; cobalt base alloys, for example an alloy consisting of 62% cobalt, 27% chromium, 5% molybdenum, 3% nickel, 1% iron and small amounts of manganese, silicon and carbon; and iron base alloys containing about 1220% aluminum. All alloy constituent percentages are expressed in this application and in the appended claims as percentages by weight.

For certain special applications, such as brazing jigs and grid mandrels, the alloys utilized should also be able to withstand the action of molten copper, molten Monel metal, molten copper-containing brazes, and other molten brazes consisting of molten metals and molten alloys generally having a melting point below 1400 C. Preferably, alloys used for these applications should not be attacked by molten brazes, should have a useful life of about 10,000 heating cycles in a reducing atmosphere such as line hydrogen, and should not be subject to spalling or reduction during use.

The alloys which can best withstand the action of molten copper, or of molten brazes generally, are those alloys which have a stable oxide coating over their surface. It is believed that this metal oxide coating prevents wetting of these alloys by molten copper or other molten brazes. It is known that articles made of aluminum or of high-aluminum alloys are covered with a thin tenacious film of aluminum oxide. Articles made of iron-aluminum alloys containing about to 20 Weight percent aluminum are similarly covered with a thin film of aluminum oxide, and are thus able to Withstand the action of molten copper or of molten brazes. However, such alloys of iron and aluminum are extremely diflicult to machine, and cannot be readily cold-worked. Since the ditliculty of working increases with increasing aluminum content, attempts have been made to utilize iron-aluminum alloys which contain less than 10% aluminum. See for example Ductile Fe-Al Alloys by E. R. Morgan and V. F. Zackay, Metal Progress, October 1955, pp. 126-128. Unfortunately, although the workability and room temperature ductility of the alloys is thus increased, these lowaluminum alloys become susceptible to spalling and to wetting by molten copper and molten brazes generally. Furthermore, such low aluminum alloys do not retain sufficient tensile strength at elevated temperatures. Accordingly, the iron-aluminum alloys of the prior art have not been utilized for the applications mentioned above.

3,085,325; Patented Apr. 16, 1953 The compositions which have been most used for these high temperature applications are generally alloys containing about one-fifth to one-fourth chromium by weight. The alloy consisting of about 20 percent chromium and percent nickel has been considered one of the best known to the prior art. This alloy is covered by a thin film of chromium oxide, which is somewhat resistant to the action of molten copper. However, these chromium alloys have been unsatisfactory in several important respects. First, in a reducing atmosphere these chromium alloys are wetted and attacked by relatively large amounts of molten copper. Second, when utilized at elevated temperatures in reducing ambients these alloys are attacked even by relatively small amounts of molten copper and molten brazes. chromium oxide on these alloys is susceptible to reduction to metallic chromium when employed in reducing ambients, such as line hydrogen. In some applications it may be necessary to reform the chromium oxide layer after each time that the article made of this chromiumnickel alloy is heated in a reducing ambient. Fourth, such alloys containing a high chromium content are relatively expensive. Fifth, chromium and nickel are strategic materials which are not readily available within the United States in emergencies.

Accordingly, it is a general object of this invention to provide improved alloys for high temperature applications.

Another object of the invention is to provide improved alloys which will exhibit good strength at elevated temperatures.

Still another object is to provide improved alloys which will maintain their strength during thousands of temperature cycles.

But another object is to provide improved alloys which will maintain dimensional stability during thousands of temperature cycles.

Yet another object is to provide improved high temperature alloys which are less expensive than alloys containing large amounts of chromium and nickel.

A further object of the invention is to provide improved high-temperature alloys which consist largely of nonstrategic metals available within the United States.

An additional object is to provide an improved high temperature alloy which is resistant to the action of molten copper and molten brazes.

Another object is to provide improved high temperature alloys which are resistant to spalling when heated in reducing atmospheres.

Still another object is to provide improved alloys which are resistant to reduction when heated in reducing atmospheres.

It is also an object of this invention to provide improved alloys which are suitable for electrical resistance heating elements.

These and other objects of the invention are attained by providing a temperature resistant iron base alloy consisting of 6 to 20 Weight percent aluminum, 1 to 15 weight percent of at least one element selected from the group consisting of molybdenum nickel, cobalt, tungsten, vanadium, niobium, tantalum, and titanium, and .01 to 1.0 Weight percent of at least one element selected from the group consisting of zirconium and hafnium, the balance being iron. It has unexpectedly been found that the addition of small amounts, not exceeding 1 weight percent, of zirconium or hafnium or both to iron-aluminum alloys greatly improves the cold workability and the machinability of the alloys, as well as increasing the recrystallization temperature of these alloys, so that their crystal size is reduced and room temperature ductility is increased.

The composition of some high temperature alloys ac- Third, the thin protective coating of cording to the invention, and the melting point of each composition, is set forth in Table I.

Table I Composition in Weight Percent Melting Point. C- Extrapolatcd from optical reading Zirco- Additive nium Aluminum lOMo

P-IQHHHH vaq soooomcooocahmmooo It will be noted that the alloysaccording to the in- .vention may contain zirconium or hafnium as in alloys 3 and 5, or both, as in alloy 4. The alloys may also contain more than one of the strengthening additives from the group consisting of Mo, Ni, Co, W, V, Nb, Ta, and Ti.

While the exact mechanism of the invention is not certain, it is believed that zirconium and hafnium may act as nucleating agents to promote grain growth, and to relieve lattice strains by changing lattice parameters. It should be noted that while zirconium and hafnium increase the workability of Fe-Al alloys when added in amounts less than 1 weight percent, they are harmful if added in amounts greater than 1 weight percent. The small amounts of zirconium and hafnium utilized also tend to strengthen the Fe-Al alloys.

The preparation of an alloy according to the invention will now be described. The aluminum used is in the form of pellets, and is 99.99% pure. The iron is granulated low-carbon electrolytic material, and is also about 99.99% pure. The zirconium and hafnium are in sponge form. In this example, the alloy charge consists of 1050 grams pellet aluminum, 12,276 grams granulated iron, and 1500 grams powdered molybdenum which has been pressed into a compact and sintered at about2000 C.

The charge is placed in a magnorite-lined refractory crucible. As the volume of the charge is larger than the internal volume of the crucible, the excess charge is contained in a cylinder of low carbon sheet iron. The cylinder becomes part of the iron fraction of the alloy. In this example, the sheet iron cylinder weighs 159 grams.

The charge is now heated by induction heating in a vacuum furnace at about 1500 C. to melt the charge. The pressure in the furnace is maintained at about 1 to 10 microns Hg during this heating cycle. Now 15 grams of sponge zirconium are added to the melt without breaking the vacuum, and the temperature of the furnace is raised to about 16-O0 C. for about 6 minutes. The furnace is then filled with argon to a pressure of about /3 atmosphere, and the molten change is poured in an atmosphere of argon into a graphite mold.

After the ingot has cooled, it is stripped from the mold and cleaned by grinding to remove burrs and surface imperfections. The end of the ingot which contains the sprue is cropped. The ingot remaining is essentially single phase in character, and is about 1%" in diameter and 20" long in this example. In order to relieve the residual stresses and strains caused by unequal cooling, the ingot is heated for 12 hours at 1 l C. in a hydrogen atmosphere. This treatment also homogenizes the ingot, in that any small amount of second phase material which may be present is thereby distributed uniformly throughout the ingot.

- The subsequent treatment of the ingot depends on the conditions.

nature of the article being fabricated. For example, when the end product desired is precision tubing, the ingot is hot rolled to the desired diameter, then rifle drilled, and tfinally cold drawn to the correct size. Such tubing may be utilized to form precision jigs which will be resistant to heat cycling in a reducing ambient, since the aluminum oxide film on the alloys of the invention is not reduced by dry hydrogen even at temperatures as high as 1130 C. In contrast, the chromium oxide film on the nickelchromium alloys of the prior art is reduced under these The jig will also be resistant to Wetting by molten copper, molten Monel metal, and molten brazes generally. Alloys numbers 1 and 2 in Table I above have been found particularly satisfactory for such tubing applications. Alloy number 15 is similarly useful, and exhibits excellent room temperature ductility. Alloy number 16 exhibits good tensile strength, good fatigue strength, and good creep strength, as well as excellent resistance to oxidation. It may be utilized for such applications as die casting.

When the article to be manufactured is an electrical resistance heating element, or a brazing mandrel, the ingot is hot swaged at 1000 C. to reduce its diameter from 1%" to 150 mils.

Thereafter the material is cold drawn to form a rod or wire of the desired diameter. Alloys according to the invention have thus been formed into rods and wires, and the resulting wires have been fabricated into electrical resistance heating elements. Such heating units have been tested by intermittently passing sufficient current through them to raise their temperature to 1177 C. (The temperatures have been estimated from optical measurements.) The heating units according to the invention were placed on life test by cycling them to a temperature of 1177 C. in air.

For comparison, a similar resistance heating unit composed of weight percent nickel-20 weight percent chromium was placed on the same life test. The prior art nickel-chromium resistance heating unit broke down after 193 hours. In contrast, the electrical resistance heating units composed of the alloys of this invention remained operative until the life test was discontinued after 283 hours. An example of a satisfactory alloy for this purpose is alloy number 13 in Table I above. If desired, a small amount of boron, less than 0.1 weight percent, may be added to improve the creep strength of the alloys.

According to the invention, an improved method of brazing metal objects utilizes the circumstances that the alloys of the invention are not wetted by molten copper or molten brazes generally. The method comprises the steps of fabricating a mandrel of an alloy which is nonwettable by molten brazes, said alloy consisting of 6 to 20 weight percent aluminum, 1 to 15 weight percent of at least one element selected from the group consisting of molybdenum, nickel, vanadium, niobium, tantalum, and titanium, .01 to 1.0 weight percent of at least one element selected from the group consisting of zirconium and hafnium, and balance iron; mounting the metal objects to be brazed on the mandrel; and bonding the aforesaid metal articles in a reducing atmosphere by means of a braze which consists essentially of metals or alloys having a melting point below 1400 C. The advantage of this method is that the molten brazes do not wet the mandrel according to this invention, and hence the metal articles thus bonded do not stick to the mandrel.

The brazing method according to the invention can be advantageously utilized to fabricate parts for electron discharge tubes, as in the following example. An ingot having the composition corresponding to alloy number 2 in Table I is prepared, heated to relieve internal stresses and strains, hot swaged to mils diameter, then cold drawn to form a rod or Wire of 66 mils diameter. The rod thus formed is heated for two hours at 1125 C.

in line hydrogen, which contains a small amount of oxygen and water vapor. The surface of the rod is thereby coated with a tenacious gray film of aluminum oxide. This aluminum oxide coating is not wetted by molten copper or by molten brazes generally, and hence is particularly suitable for brazing mandrels.

The oxided rod is positioned in a grid-winding lathe, and a plurality of fine wires are stretched longitudinally along the surface of the rod. The longitudinal wires, which in this example are made of nickel-plated molybdenum and are only 0.8 mill in diameter, are all parallel to the rod, and do not intersect each other.

Next another fine wire is wrapped in a tight helix around the rod and the longitudinal wires. This helical wire thus crosses the longitudinal wires at many points along their length. In this example, the helical wire is composed of copper-plated molybdenum and is 0.8 mil in diameter. The coppercoating of the helical wire serves as the braze in this example. The assemblage of the oxided rod, which serves as the brazing mandrel, the longitudinal wires, and the helical wire is heated in a reducing atmosphere to a temperature somewhat above the melting point of copper. Heating the assemblage to 1175" C. for less than 1 minute has been found satisfactory. The copper plating on the helical wire softens during this step, and adheres to the longitudinal wires at all the points of intersection. The copper actually melts and forms a eutectic with the nickel coating on the longitudinal wires, the composition of this copper-nickel eutectic being similar to Monel metal. The bonded wires thus form a hollow tubular cage structure. During this step the oxided rod expands, so that the helical wire and the plurality of longitudinal wires are under tension while they are being bonded. Thereafter the assemblage is cooled to room temperature, and the oxided rod which serves as the brazing mandrel contracts. It thus becomes easy to slide the wire cage structure otf the mandrel. This wire cage is then cut into suitable lengths and used for such applications as the grids of high frequency triode tubes.

When the mandrel utilized consists of prior art materials, such as nickel-chromium alloys and the like, the molten copper or other molten brazes utilized to bond the fine metal wires tends to wet the mandrel, and hence tends to stick to the mandrel when the assemblage is cooled. It therefore becomes very diflicult to remove the wire cage from the mandrel without breaking or injuring the cage. A high scrap rate thus results. An important advantage of the instant method of brazing is that the mandrels according to the invention are not wetted or attacked by molten copper, molten Monel metal, or molten brazes generally. Metal articles which have been brazed according to the invention as described above are readily removed from the brazing mandrels according to the invention without damage, and the scrap rate in the fabrication of metal structures such as vacuum tube grids is considerably decreased.

There have thus been described improved alloys which are resistant to elevated temperatures and to molten brazes, as well as improved brazing methods utilizing the aforesaid alloys.

What is claimed is:

1. The method of brazing metal objects comprising the steps of fabricating a mandrel of an alloy which is nonwettable by molten copper-containing brazes, said alloy consisting of 6 to 20 weight percent aluminum, 1 to 15 weight percent of at least one element selected from the group consisting of molybdenum, nickel, vanadium, niobium, tantalum and titanium, .01 to 1.0 weight percent of at least one element selected from the group consisting of zirconium and hafnium, and balance iron; mounting the metal objects to be brazed on the aforesaid mandrel; and bonding the aforesaid metal objects to each other by means of a copper-containing braze, said braze having a melting point below 1400 C.

2. The method as in claim 1, in which said bonding step is performed in a reducing atmosphere.

3. The method as in claim 1, in which said alloy contains 1 to 15 weight percent molybdenum.

4. The method of brazing metal objects comprising the steps of fabricating a mandrel of an alloy which is non-wettable by molten copper-containing brazes, said alloy consisting of 8 weight percent aluminum, 10 weight percent molybdenum, 0.25 Weight percent zirconium, and balance iron; mounting the metal objects to be brazed on the aforesaid mandrel; and bonding the aforesaid metal objects to each other by means of a copper-containing braze, said braze having a melting point below 1400 C.

References Cited in the file of this patent UNITED STATES PATENTS Flintermann Sept. 6, 1927 Hansen Mar. 7, 1950 OTHER REFERENCES 

1. THE METHOD OF BRAZING METAL OBJECTS COMPRISING THE STEPS OF FABRICATING A MANDREL OF AN ALLOWY WHICH IS NONWETTABLE BY MOLTEN COPPER-CONTAINING BRAZES, SAID ALLOY CONSISTING OF 6 TO 20 WEIGHT PERCENT ALUMINUM, 1 TO 15 WEIGHT PERCENT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, NICKEL, VANADIUM, NIOBIUM, TANTALUM AND TITANIUM, .01 TO 1.0 WEIGHT PERCENT OF AT LEAST ONE ELEMENT SELECTED ROM THE GROUP CONSISTING OF ZIRCONIUM AND HAFNIUM, AND BALANCE IRON; MOUNTING THE METAL OBJECTS TO BE BRAZED ON THE AFORESAID MANDREL; AND BONDING THE AFORESAID METAL OBJECTS TO EACH OTHER BY MEANS OF A COPPER-CONTAINING BRAZE, SAID BRAXE HAVING A MELTING POINT BELOW 1400*C. 